We report on a colossal negative magnetoresistance (MR) in a GaAs/AlGaAs quantum well which, at low temperatures, is manifested by a drop of the resistivity by more than an order of magnitude at a magnetic field B≈1 kG. In contrast to MR effects discussed earlier, the MR reported here is not parabolic, even at small B, and persists to much higher in-plane magnetic fields and temperatures. Remarkably, the temperature dependence of the resistivity at B≈1 kG is linear over the entire temperature range studied (from 1 to 30 K) and appears to coincide with the high-temperature limit of the zero-field resistivity, hinting on the important role of acoustic phonons.